Extra-cardiovascular cardiac pacing system

ABSTRACT

An extra-cardiovascular medical device is configured to select a capacitor configuration from a capacitor array and deliver a low voltage, pacing pulse by discharging the selected capacitor configuration across an extra-cardiovascular pacing electrode vector. In some examples, the medical device is configured to determine the capacitor configuration based on a measured impedance of the extra-cardiovascular pacing electrode vector.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Division of U.S. patent application Ser. No.14/957,651, filed Dec. 3, 2015, entitled “EXTRA-CARDIOVASCULAR CARDIACPACING SYSTEM,” the content of which is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a system, device and method for delivering cardiac pacingpulses using extra-cardiovascular electrodes.

BACKGROUND

A variety of implantable medical devices (IMDs) for delivering atherapy, monitoring a physiological condition of a patient or acombination thereof have been clinically implanted or proposed forclinical implantation in patients. Some IMDs employ one or moreelongated electrical leads carrying stimulation electrodes, senseelectrodes, and/or other sensors. IMDs may deliver therapy to or monitorconditions of a variety of organs, nerves, muscle or tissue, such as theheart, brain, stomach, spinal cord, pelvic floor, or the like.Implantable medical leads may be configured to position electrodes orother sensors at desired locations for delivery of electricalstimulation or sensing of physiological conditions. For example,electrodes or sensors may be carried along a distal portion of a leadthat is extended subcutaneously, transvenously, or submuscularly. Aproximal portion of the lead may be coupled to an implantable medicaldevice housing, which contains circuitry such as signal generationcircuitry and/or sensing circuitry.

Some IMDs, such as cardiac pacemakers or implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to theheart of the patient via electrodes carried by one or more implantableleads and/or the housing of the pacemaker or ICD. The leads may betransvenous, e.g., advanced into the heart through one or more veins toposition endocardial electrodes in intimate contact with the hearttissue. Other leads may be non-transvenous leads implanted outside theheart, e.g., implanted epicardially, pericardially, or subcutaneously.The electrodes are used to deliver electrical stimulation pulses to theheart to address abnormal cardiac rhythms.

IMDs capable of delivering electrical stimulation for treating abnormalcardiac rhythms typically sense signals representative of intrinsicdepolarizations of the heart and analyze the sensed signals to identifythe abnormal rhythms. Upon detection of an abnormal rhythm, the devicemay deliver an appropriate electrical stimulation therapy to restore amore normal rhythm. For example, a pacemaker or ICD may deliver lowvoltage pacing pulses to the heart upon detecting bradycardia ortachycardia using endocardial or epicardial electrodes. An ICD maydeliver high voltage cardioversion or defibrillation shocks to the heartupon detecting fast ventricular tachycardia or fibrillation usingelectrodes carried by transvenous leads or non-transvenous leads. Thetype of therapy delivered and its effectiveness in restoring a normalrhythm depends at least in part on the type of electrodes used todeliver the electrical stimulation and their location relative to hearttissue.

SUMMARY

In general, the disclosure is directed to techniques for deliveringextra-cardiovascular cardiac pacing pulses. A pacemaker or ICD operatingaccording to the techniques disclosed herein measures a pacing electrodevector impedance and determines a capacitor configuration based on themeasured impedance and pacing pulse duration. The capacitorconfiguration is selected so that an RC time constant of the dischargingcapacitor configuration across the pacing electrode vector results in apacing pulse having a truncated pulse amplitude that is greater than athreshold amplitude. Extra-cardiovascular pacing may be delivered at apacing pulse amplitude below a pain threshold of the patient with apulse width long enough to deliver adequate energy to successfully pacethe heart.

In one example, the disclosure provides an extra-cardiovascular medicaldevice including an impedance measurement module configured to measurean impedance of an extra-cardiovascular pacing electrode vector whenextra-cardiovascular pacing electrodes are electrically coupled to themedical device. The medical device includes a therapy delivery modulehaving a capacitor array for producing a pacing pulse and a pacingcontrol module coupled to the therapy delivery module and the impedancemeasurement module. The pacing control module is configured to controlthe impedance measurement module to measure the impedance of theextra-cardiovascular pacing electrode vector, determine a capacitorconfiguration based on the measured impedance, and control the therapydelivery module to deliver a cardiac pacing pulse by discharging thecapacitor configuration for a predetermined pulse width across theextra-cardiovascular pacing electrode vector.

In another example, the disclosure provides a method performed by anextra-cardiovascular medical device including measuring an impedance ofan extra-cardiovascular pacing electrode vector whenextra-cardiovascular pacing electrodes are electrically coupled to themedical device, determining a capacitor configuration based on themeasured impedance, and delivering a cardiac pacing pulse by dischargingthe first capacitor configuration for a predetermined pulse width acrossthe extra-cardiovascular pacing electrode vector.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control module of an extra-cardiovascular medicaldevice, cause the medical device to measure an impedance of anextra-cardiovascular pacing electrode vector when extra-cardiovascularpacing electrodes are electrically coupled to the medical device,determine a capacitor configuration based on the measured impedance, anddeliver a cardiac pacing pulse by discharging the capacitorconfiguration for a predetermined pulse width across theextra-cardiovascular pacing electrode vector.

In another example, the disclosure provides an extra-cardiovascularmedical device including a therapy delivery module having a capacitorarray including a plurality of capacitors for producing a pacing pulseand a pacing control module coupled to the therapy delivery module. Thepacing control module is configured to select a first capacitorconfiguration comprising a first combination of the plurality ofcapacitors and control the therapy delivery module to select the firstcapacitor configuration by selectively enabling the first combination ofthe plurality of capacitors of the capacitor array. The pacing controlmodule is configured to start a pulse width timing interval and controlthe therapy delivery module to start delivery of a cardiac pacing pulseby discharging the first capacitor configuration across anextra-cardiovascular pacing electrode vector coupled to the therapydelivery module. The pacing control module is further configured toobtain a sampled amplitude of the cardiac pacing pulse during the pulsewidth timing interval, compare the sampled amplitude to an amplitudethreshold, and control the therapy delivery module to select a secondcapacitor configuration comprising a second combination of the pluralityof capacitors different than the first combination and start dischargingthe second capacitor configuration across the extra-cardiovascularpacing electrode vector before the pulse width timing interval expiresin response to the amplitude of the cardiac pacing pulse being less thanor equal to the amplitude threshold.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a patient implanted with an exampleextra-cardiovascular IMD system that includes a subcutaneously implantedIMD coupled to an extra-cardiovascular sensing, pacing andcardioversion/defibrillation (CV/DF) lead.

FIG. 2 is a transverse view of a patient depicting an alternative,substernal location of the extra-cardiovascular lead of FIG. 1.

FIG. 3 is a schematic diagram of the IMD of FIG. 1 according to oneexample.

FIG. 4 is a schematic diagram of a pacing control module and a therapydelivery module included in IMD 14.

FIG. 5 is a schematic diagram of the pacing control module of FIG. 4.

FIG. 6 is a schematic diagram of a capacitor selection and controlmodule included in the therapy delivery module of FIG. 4.

FIG. 7A is a conceptual diagram of a pacing pulse generated by thetherapy delivery module of FIG. 4.

FIG. 7B is a conceptual diagram of a pacing pulse that may be deliveredby the IMD of FIG. 1 using a capacitor configuration that is adjustedduring the pacing pulse delivery.

FIG. 7C is a conceptual diagram of one example of a pacing pulse thatmay be delivered by the IMD of FIG. 1 using an adjusted capacitorconfiguration during the pacing pulse when a pacing electrode vectorimpedance change occurs.

FIG. 8 is a conceptual diagram of a look-up table stored in memory ofthe IMD of FIG. 1.

FIG. 9 is a flow chart of a method performed by the IMD of FIG. 1 fordelivering a pacing pulse according to one example.

FIG. 10 is a flow chart that corresponds to operations performed in theflow chart of FIG. 9 for delivering a pacing pulse.

FIG. 11 is a flow chart of a pacing control method according to anotherexample.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for delivering lowvoltage pacing pulses using extra-cardiovascular electrodes that are notdirectly contacting cardiac tissue. As used herein, the term“extra-cardiovascular” refers to a position outside the blood vessels,heart, and pericardium surrounding the heart of a patient. Implantableelectrodes carried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but not in intimatecontact with myocardial tissue. The term “low voltage” in reference toextra-cardiovascular pacing pulses refers to a voltage level that isbelow a pain threshold of a patient and may be on the order of 20 V orless in some examples.

Cardiac pacing is commonly delivered using electrodes in close orintimate contact with myocardial tissue such as endocardial electrodesor epicardial electrodes. Pacing pulses delivered using endocardialelectrodes are typically up to a maximum of 8 V in pulse amplitude witha pulse width of 2.0 ms or less. A pacing pulse that successfully pacesthe heart might be 2.5 V in amplitude with a 0.5 ms pulse width, forexample. The pulse amplitude and pulse width are selected to provide apacing pulse having adequate energy to capture the heart, e.g., causedepolarization of the ventricles of the heart.

ICD systems have been proposed or are commercially available thatutilize electrodes carried by subcutaneous or submuscular leads to senseelectrocardiogram (ECG) signals and deliver high-energy shocks tocardiovert or defibrillate the heart. Electrodes carried by subcutaneousor submuscular leads may be used for delivering high-voltage,short-duration pulses during a post-shock recovery phase to treatasystole that may occur post-shock. Discomfort caused by these highvoltage pacing pulses may be deemed acceptable post-shock in light ofthe life-saving treatment provided by shock delivery and post-shock,high-voltage pacing.

Such electrodes positioned subcutaneously or submuscularly are generallynot used for delivering pacing therapies that are sustained over longertime periods or for conditions that are not immediately lifethreatening, e.g., for bradycardia pacing, anti-tachycardia pacing(ATP), or cardiac resynchronization therapy (CRT). The relatively highvoltage amplitude required to successfully capture the heart when theelectrodes are not in close contact with the myocardium may causeintolerable or unacceptable pain or discomfort to the patient. The highvoltage is required in order to deliver enough energy within a limitedpacing pulse width, e.g., 2 ms or less. This maximum pacing pulse widthis limited by the decay rate of the pacing pulse which is dependent onthe capacitance of the capacitor being discharged to deliver the pacingpulse and the impedance of the pacing electrode vector through which thecapacitor is discharged

For a given pulse width typically used with endocardial or epicardialelectrodes, e.g., less than 2 ms, the pulse amplitude required tocapture the heart using the same pulse width when pacing withextra-cardiovascular electrodes, such as subcutaneous or submuscularelectrodes, may cross an acceptable pain threshold. A pacing pulsehaving a lower voltage amplitude that is not painful to the patient whendelivered by extra-cardiovascular electrodes may require a relativelylong pulse width that is likely to be beyond the capacity of a typicallow voltage pacing capacitor due to the relatively fast decay rate ofthe pulse amplitude. A low voltage pacing capacitor may have acapacitance of 10 microfarads or less. Since a pacing pulse is deliveredas the pacing capacitor is discharged across the pacing electrodevector, the pacing pulse amplitude may decay below an effective voltageamplitude before the required pacing pulse width is reached forsuccessfully delivering the pacing pulse energy required to capture andpace the heart.

As disclosed herein, an implantable, extra-cardiovascular medical devicesystem includes a therapy delivery module having an array of capacitorsthat are controlled by a pacing control module for delivering lowvoltage pacing pulses having a pulse width that is long enough, e.g.,greater than 1.5 ms, to successfully pace the heart using a pulseamplitude that is below a pain threshold of the patient withoutrequiring electrodes in direct contact with the myocardial orpericardial tissue. The pacing control module controls the decay rate ofthe pacing pulse by selecting a capacitor configuration that maintainsthe amplitude of the pacing pulse above a target amplitude for theduration of the pulse width. The techniques disclosed herein may beimplemented in any implantable pacemaker or ICD and particularly in apacemaker or ICD having extra-cardiovascular electrodes. The electrodesmay be carried by a medical electrical lead extending from the pacemakeror ICD and/or carried by the housing of the pacemaker or ICD. Thetechniques disclosed herein are not necessarily limited to implantablesystems and may be implemented in an external pacemaker or ICD usingcutaneous surface electrodes or transcutaneous electrodes.

FIG. 1 is a conceptual diagram of a patient 12 implanted with anextra-cardiovascular IMD system 10 that includes a subcutaneouslyimplanted IMD 14 coupled to an extra-cardiovascular sensing, pacing andcardioversion/defibrillation (CD/DF) lead 16. IMD 14 includes a housing15 and connector assembly 17. IMD 14 acquires cardiac electricalsignals, e.g., ECG signals, using electrodes carried by lead 16 and maybe configured to deliver pacing pulses using extra-cardiovascularelectrodes carried by lead 16. As will be described herein, IMD 14includes a pacing control module that controls an array of pacingcapacitors for delivering pacing pulses via extra-cardiovascularelectrodes. The pacing pulses have a pulse amplitude that is less thanthe pain threshold of the patient and a pulse width that is long enough,e.g., greater than 1.5 ms or greater than 2.0 ms, to successfully pacethe heart using extra-cardiovascular electrodes without causingunacceptable pain or discomfort to the patient. The cardiac electricalsignals received by IMD 14 are used for determining the patient's heartrhythm and providing appropriate pacing therapy as needed, such asbradycardia pacing or ATP. IMD 14 is configured as an ICD in thisexample, capable of detecting shockable rhythms and delivering a CV/DFshock therapy via defibrillation electrode 24 carried by lead 16. Inother examples, IMD 14 may be configured as a pacemaker for deliveringlow voltage pacing therapies without high voltage CV/DF shock therapycapability. In this case, lead 16 may be carry pacing and sensingelectrodes, e.g., electrodes 28 and 30, without incorporating adefibrillation electrode 24 or defibrillation electrode 24 may beincluded and used as a return anode during cardiac pacing usingelectrode 28 or 30 as a cathode.

Lead 16 includes a proximal end 27 that is connected to IMD 14 and adistal portion 25 that carries electrodes 24, 28 and 30. Electrode 24 isa defibrillation electrode that may be used in combination with theconductive housing 15 of IMD 14 for delivering high voltage CV/DFshocks. All or a portion of housing 15 of IMD 14 may be formed of aconductive material, such as titanium or titanium alloy, and coupled tointernal IMD circuitry to function as an electrode, sometimes referredto as a “CAN electrode.” A shock vector pathway extends fromdefibrillation electrode 24 to housing 15, through the ventricularmyocardium. Defibrillation electrode 24 is typically an elongated coilelectrode having a relatively higher surface area than electrodes 28 and30 but may be implemented as another type of electrode other than a coilelectrode.

Electrodes 28 and 30 are referred to herein as pacing and sensingelectrodes because they generally are used for delivering pacing pulsesand sensing cardiac electrical signals. An ECG signal may be acquiredusing any combination of electrodes 28, 30 and housing 15. For example,IMD 14 may sense cardiac electrical signals using a sensing electrodevector between electrodes 28 and 30, a sensing electrode vector betweenelectrode 28 and housing 15 or a sensing electrode vector betweenelectrode 30 and housing 15 may be chosen. In some examples, a sensingelectrode vector may even include defibrillation electrode 24, e.g., inconjunction with one or more of electrodes 28, 30, or housing 15. IMD 14may include more than one sensing channel such that electrode sensingvectors may be selected two at a time by IMD 14 for monitoring for ashockable rhythm or a need for cardiac pacing.

Pacing pulses may be delivered using any combination of electrodes 24,28, 30 and housing 15. The pacing electrode vector selected fordelivering pacing pulses may be selected based on pacing electrodevector impedance measurements and capture threshold testing. Forexample, a pacing electrode vector may be selected from among electrodes24, 28, 30 and housing 15 that has the lowest impedance and/or thelowest pulse width that captures the heart for a programmed pacing pulseamplitude. The pacing pulse amplitude may be programmed to be below athreshold for pain and discomfort, which may be based on individualpatient testing and/or clinical data.

While three electrodes 24, 28 and 30 are shown along lead 16, lead 16may carry more or fewer electrodes in other examples. In the exampleillustrated in FIG. 1, pacing and sensing electrodes 28 and 30 areseparated from one another by defibrillation electrode 24. In otherwords, sensing electrode 28 is located distal to defibrillationelectrode 24, and sensing electrode 30 is proximal to defibrillationelectrode 24. In various examples, electrodes 28 and 30 may be carriedalong lead 16 at other locations than those shown but are generallypositioned to acquire cardiac electrical signals having acceptablecardiac signal strength for sensing cardiac events, such as R-wavesignals that occur upon depolarization of the ventricles and fordelivering low voltage pacing pulses for successfully capturing thepatient's heart 26. Pacing pulses may be delivered using any combinationof electrodes 24, 28, 30 and/or housing 15, e.g., using electrodes 28and 30 in a bipolar pair, using one of electrodes 28 or 30 paired withhousing 15, using one of electrodes 28 or 30 paired with defibrillationelectrode 24, using both or electrodes 28 and 30 tied together as amulti-polar cathode electrode paired with housing 15 or withdefibrillation electrode 24, and so on.

Two or more electrodes used for delivering low voltage pacing pulses maybe located at different locations along lead 16 in other examples. Forinstance, an electrode configuration including two pacing and sensingelectrodes that are spaced apart along lead body 18 adjacent to eachother without an intervening defibrillation electrode 24 may be used fordelivering low voltage pacing pulses as disclosed herein. Two pacing andsensing electrodes may be positioned adjacent to each other at spacedapart locations in between two defibrillation electrodes as generallydisclosed in commonly-assigned U.S. patent application Ser. No.14/519,436 and U.S. patent application Ser. No. 14/695,255, both ofwhich are incorporated herein by reference in their entirety.

In other examples, an extra-cardiovascular lead may include multipledefibrillation electrode segments, and multiple pacing and sensingelectrodes may be disposed between the defibrillation electrodesegments. The defibrillation electrode segments and pacing and sensingelectrodes, which may be ring electrodes, may be carried by anundulating or zig-zagging distal portion of the lead body as generallydisclosed in provisionally-filed U.S. Pat. Application No. 62/089,417,and may be utilized in conjunction with the pacing techniques disclosedherein. U.S. Pat. Application No. 62/089,417 is also incorporated hereinby reference in its entirety. In still other examples, lead 16 may carrya single pace/sense electrode to serve as a pacing cathode electrodewith housing 15 or with a defibrillation electrode 24 or any of thedefibrillation electrodes or defibrillation electrode segments shown anddescribed in the above-incorporated references, serving as the returnanode electrode.

In other examples, dedicated pacing electrodes and separate, dedicatedsensing electrodes may be carried by lead 16 or another lead coupled toIMD 14. It is understood that one or more leads may be coupled to IMD 14for connecting at least one pacing and sensing electrode to IMD 14 formonitoring cardiac electrical signals, and delivering low voltage pacingpulses and at least one defibrillation electrode and for deliveringCV/DF shock therapy when IMD 14 is configured as an ICD. Pacingtherapies that may be delivered by IMD 14 may include bradycardiapacing, ATP, CRT and/or post-shock pacing for treating bradycardia orasystole after a CV/DF shock.

Lead 16 is illustrated in FIG. 1 as being implanted at least partiallyin a substernal location, e.g., between the heart 26 and ribcage 32 orsternum 22. In one such configuration, the proximal portion of lead 16extends subcutaneously from IMD 14 (which is implanted near amidaxillary line on the left side of patient 12) toward sternum 22. At alocation near xiphoid process 20, lead 16 bends or turns superiorly anddistal portion 25 of lead 16, which carries electrodes 24, 28 and 30,extends substernally, under or below the sternum 22 in the anteriormediastinum 36.

FIG. 2 is a transverse view of patient 12 showing the distal portion 25of lead 16 extending substernally, e.g., at least partially in oradjacent to the anterior mediastinum 36. Anterior mediastinum 36 isbounded laterally by pleurae 39, posteriorly by pericardium 38, andanteriorly by sternum 22. In some instances, the anterior wall ofanterior mediastinum 36 may also be formed by the transversus thoracisand one or more costal cartilages. Anterior mediastinum 36 includes aquantity of loose connective tissue (such as areolar tissue), adiposetissue, some lymph vessels, lymph glands, substernal musculature (e.g.,transverse thoracic muscle), branches of the internal thoracic artery,the thymus gland, and the internal thoracic vein. In one example, thedistal portion of lead 16 extends along the posterior side of sternum 22substantially within the loose connective tissue and/or substernalmusculature of anterior mediastinum 36. Lead 16 may be at leastpartially implanted in other extra-cardiovascular, intrathoraciclocations, e.g., along ribcage 32 or along or adjacent to the perimeterof the pericardium 38 or within the pleural cavity.

IMD 14 may also be implanted at other subcutaneous or submuscularlocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which IMD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 22. When the IMD 14 is implanted in the pectoralregion, the system 10 may include a second lead that extends along theleft side of the patient and includes a defibrillation electrode and/orone or more pacing electrodes positioned along the left side of thepatient to function as an anode or cathode of a therapy delivery vectorincluding another electrode located anteriorly for delivering electricalstimulation to heart 26 positioned there between.

In other examples, lead 16 may be implanted at otherextra-cardiovascular locations. For instance, lead 16 may be implantedsubcutaneously or submuscularly, between the skin and the ribcage 32 orbetween the skin and sternum 22. Lead 16 extends subcutaneously from IMD14 toward xiphoid process 20 as shown in FIG. 1, but instead ofextending substernally, lead 16 may bend or turn at a location nearxiphoid process 20 and extend subcutaneously or submuscularly superior,substantially parallel to sternum 22. The distal portion 25 of lead 16may be parallel over sternum 22 or laterally offset from sternum 22, tothe left or the right. In other examples, the distal portion 25 of lead16 may be angled laterally away from sternum 22, either to the left orthe right, such that the distal portion 25 extends non-parallel tosternum 22.

In another example, IMD 14 may be implanted subcutaneously outside theribcage 32 in an anterior medial location. Lead 16 may be tunneledsubcutaneously into a location adjacent to a portion of the latissimusdorsi muscle of patient 12, from a medial implant pocket of IMD 14laterally and posterially to the patient's back to a location oppositeheart 26 such that the heart 26 is generally disposed between the IMD 14and electrodes 24, 28 and 30. The techniques disclosed herein forgenerating low voltage pacing pulses for pacing the heart usingextra-cardiovascular electrodes are not limited to a particularsubcutaneous, submuscular, supra-sternal, substernal or intra-thoraciclocation of the extra-cardiovascular electrodes.

Referring again to FIG. 1, lead 16 includes an elongated lead body 18that carries the electrodes 24, 28 and 30 and insulates one or moreelongated electrical conductors (not illustrated) that extend from arespective electrode 24, 28 and 30 through the lead body 18 to aproximal connector (not shown) that is coupled to IMD 14 at leadproximal end 27. Lead body 18 may be formed from a non-conductivematerial, such as silicone, polyurethane, fluoropolymers, or mixturesthereof or other appropriate materials, and is shaped to form one ormore lumens within which the one or more conductors extend. Theconductors are electrically coupled to IMD circuitry, such as a therapydelivery module and an electrical sensing module, via connections in IMDconnector assembly 17 that includes a connector bore for receiving theproximal connector of lead 16 and associated electrical feedthroughscrossing IMD housing 15. The electrical conductors transmit electricalstimulation therapy from a therapy delivery module within IMD 14 to oneor more of electrodes 24, 28, and 30, and transmit cardiac electricalsignals from one or more of electrodes 24, 28, and 30 to the sensingmodule within IMD 14.

Housing 15 forms a hermetic seal that protects internal electroniccomponents of IMD 14. As indicated above, housing 15 may function as a“CAN electrode” since the conductive housing or a portion thereof may beelectrically coupled to internal circuitry to be used as an indifferentor ground electrode during cardiac signal sensing or during electricalstimulation therapy delivery. As will be described in further detailherein, housing 15 may enclose one or more processors, memory devices,transmitters, receivers, sensors, sensing circuitry, therapy circuitryand other appropriate components.

The example IMD system 10 of FIG. 1 is illustrative in nature and shouldnot be considered limiting of the techniques described in thisdisclosure. The techniques disclosed herein may be implemented innumerous ICD or pacemakers operating with electrode configurations thatinclude extra-cardiovascular electrodes for delivering cardiac pacingpulses. The IMD system 10 is referred to as an extra-cardiovascular IMDsystem because lead 16 is a non-transvenous lead, positioned outside theblood vessels, heart 26 and pericardium 38.

An external device 40 is shown in telemetric communication with IMD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with IMD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenIMD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from IMD 14 and to programoperating parameters and algorithms in IMD 14 for controlling IMDfunctions. External device 40 may be used to program cardiac rhythmdetection parameters and therapy control parameters used by IMD 14. Datastored or acquired by IMD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from IMD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand-held device.

FIG. 3 is a schematic diagram of IMD 14 according to one example. Theelectronic circuitry enclosed within housing 15 includes software,firmware and hardware that cooperatively monitor one or more cardiacelectrical signals, determine when a pacing therapy is necessary, anddeliver prescribed pacing therapies as needed. When IMD 14 is configuredas an ICD as illustrated herein, the software, firmware and hardware isalso configured to determine when a CV/DF shock is necessary and deliverprescribed CV/DF shock therapies. IMD 14 may be coupled to a lead, suchas lead 16 shown in FIG. 1, carrying extra-cardiovascular electrodes 24,28 and 30, for delivering pacing therapies, CV/DF shock therapies andsensing cardiac electrical signals.

IMD 14 includes a control module 80, memory 82, therapy delivery module84, electrical sensing module 86, telemetry module 88, impedancemeasurement module 90 and an optional sensor module 92. A power source98 provides power to the circuitry of IMD 14, including each of themodules 80, 82, 84, 86, 88, 90 and 92 as needed. Power source 98 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. Power source 98 is coupledto low voltage (LV) and high voltage (HV) charging circuits included intherapy delivery module 84 for charging LV and HV capacitors,respectively, included in therapy delivery module 84 for generatingtherapeutic electrical stimulation pulses.

The functional blocks shown in FIG. 3 represent functionality includedin IMD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to IMD 14 herein. For example, themodules may include analog circuits, e.g., amplification circuits,filtering circuits, and/or other signal conditioning circuits. Themodules may also include digital circuits, e.g., analog-to-digitalconverters, digital signal processors (DSPs), combinational orsequential logic circuits, integrated circuits, application specificintegrated circuits (ASICs), memory devices, etc. As used herein, theterm “module” refers to an ASIC, an electronic circuit, a processor(shared, dedicated, or group) and memory that execute one or moresoftware or firmware programs, a combinational logic circuit, statemachine, or other suitable components that provide the describedfunctionality. The particular form of software, hardware and/or firmwareemployed to implement the functionality disclosed herein will bedetermined primarily by the particular system architecture employed inthe IMD and by the particular detection and therapy deliverymethodologies employed by the IMD. Providing software, hardware, and/orfirmware to accomplish the described functionality in the context of anymodern IMD system, given the disclosure herein, is within the abilitiesof one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory, computer-readable media storing instructions that, whenexecuted by one or more processing circuits, cause control module 80 orother IMD modules to perform various functions attributed to IMD 14 orthose IMD modules. The non-transitory computer readable media storingthe instructions may include any of the media listed above.

The functions attributed to the modules herein may be embodied as one ormore processors, hardware, firmware, software, or any combinationthereof. Depiction of different features as modules is intended tohighlight different functional aspects and does not necessarily implythat such modules must be realized by separate hardware, firmware orsoftware components. Rather, functionality associated with one or moremodules may be performed by separate hardware, firmware or softwarecomponents, or integrated within common hardware, firmware and/orsoftware components. For example, pacing therapy control operationsperformed by control module 80 may be implemented in a processorexecuting instructions stored in memory 82.

Control module 80 communicates with therapy delivery module 84 andelectrical sensing module 86 for sensing cardiac electrical activity,detecting cardiac rhythms, and controlling delivery of cardiacelectrical stimulation therapies in response to sensed cardiac signals.Therapy delivery module 84 and electrical sensing module 86 areelectrically coupled to electrodes 24, 28, and 30 carried by lead 16(shown in FIG. 1) and the housing 15, which may function as a common orground electrode.

Electrical sensing module 86 is selectively coupled to electrodes 28, 30and housing 15 in order to monitor electrical activity of the patient'sheart. Electrical sensing module 86 may additionally be selectivelycoupled to electrode 24. Sensing module 86 is enabled to selectivelymonitor one or more sensing vectors selected from the availableelectrodes 24, 28, 30 and 15. For example, sensing module 86 may includeswitching circuitry for selecting which of electrodes 24, 28, 30 andhousing 15 are coupled to sense amplifiers or other cardiac eventdetection circuitry included in sensing module 86. Switching circuitrymay include a switch array, switch matrix, multiplexer, or any othertype of switching device suitable to selectively couple sense amplifiersto selected electrodes. The cardiac event detection circuitry withinelectrical sensing module 86 may include one or more sense amplifiers,filters, rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), or other analog or digital components.

In some examples, electrical sensing module 86 includes multiple sensingchannels for acquiring cardiac electrical signals from multiple sensingelectrode vectors selected from electrodes 24, 28, 30 and housing 15.Each sensing channel may be configured to amplify, filter and rectifythe cardiac electrical signal received from selected electrodes coupledto the respective sensing channel to improve the signal quality forsensing cardiac events, e.g., R-waves.

Each sensing channel includes cardiac event detection circuitry forsensing cardiac events from the received cardiac electrical signaldeveloped across the selected electrodes 24, 28, 30 and/or 15. Cardiacevent sensing thresholds used by each sensing channel may beautomatically adjusted according to sensing control parameters, whichmay be stored in memory 82. Each sensing channel senses a cardiac eventwhen the respectively received cardiac electrical signal crosses theauto-adjusting cardiac event sensing threshold.

Each time the received cardiac electrical signal crosses the sensingthreshold for a given channel, a cardiac sensed event signal is producedand passed to control module 80. For example, R-wave sensed eventsignals may be passed to control module 80 when a received cardiacelectrical signal crosses an R-wave sensing threshold. Sensed eventsignals produced by electrical sensing module 86 may be used by controlmodule 80 for detecting a shockable rhythm and/or for detecting a needfor pacing. For example, control module 80 may respond to sensed eventsignals by setting pacing escape intervals for controlling the timing ofpacing pulses delivered by therapy delivery module 84. In addition tothe sensed cardiac event signals, electrical sensing module 86 mayoutput a digitized ECG signal for use by control module 80 indetecting/confirming tachycardia, e.g., via a morphology or waveletanalysis.

Therapy delivery module 84 includes an LV therapy delivery module 85 fordelivering low voltage pacing pulses using an extra-cardiovascularpacing electrode vector selected from electrodes 24, 28, 30 and housing15. The LV therapy delivery module includes an array of capacitors thatare selectably controlled by control module 80 to provide low voltage,long pulse width pacing pulses having a truncated pulse amplitude atpulse termination that is greater than a threshold amplitude. LVcapacitors included in the LV therapy delivery module 85 are charged toa voltage according to a programmed pacing pulse amplitude by an LVcharging circuit (not shown in FIG. 3) included in therapy deliverymodule 84. At an appropriate time, the LV therapy delivery module 85couples a selected capacitor configuration to a pacing electrode vectorto discharge the capacitor configuration over a predetermined pacingpulse width.

As described below, a pacing control module included in control module80 may be configured to receive a feedback signal from LV therapydelivery module 85. The feedback signal indicates the amplitude of thepacing pulse as it is decaying over the pacing pulse width duringcapacitor discharge. If the pulse amplitude falls to a thresholdamplitude before the end of the pulse width, the pacing control modulemay be configured to adjust the output signal of the LV therapy deliverymodule 85 by enabling at least one additional capacitor in the capacitorarray of LV therapy delivery module 85. By enabling an additionalcapacitor(s), the voltage amplitude of the pacing pulse is maintained ator above a minimum acceptable amplitude during the relatively longdischarge period defined by the pacing pulse width. The minimumacceptable amplitude is the minimum amplitude at which capture andsuccessful pacing of the heart is highly probable.

Impedance measurement module 90 may be electrically coupled to theavailable electrodes 24, 28 and 30 and housing 15 for performingimpedance measurements of a selected pacing electrode vector. Controlmodule 80 may control impedance measurement module 90 to performimpedance measurements prior to pacing pulse delivery and/or during apacing pulse delivery. For example, control module 80 may pass a signalto impedance measurement module 90 to initiate an impedance measurementfor a selected pacing electrode vector. Impedance measurement module 90is configured to apply a drive or excitation current across a selectedpacing electrode vector and determine the resulting voltage. The voltagesignal may be used directly as the impedance measurement or impedancemay be determined from the applied current and the measured voltage. Theimpedance measurement is passed to control module 80 for use inselecting a capacitor configuration for delivering pacing pulses. Thecapacitor configuration selected based on the impedance measurement maybe the initial capacitor configuration used to deliver a pacing pulse oran adjusted capacitor configuration used to adjust the pacing pulseamplitude in real-time during a pacing pulse. Capacitor configurationdata is passed from control module 80 to LV therapy delivery module 85for use in delivering pacing pulses using the selected capacitorconfiguration as described in greater detail below.

Therapy delivery module 84 may additionally include HV therapy deliverymodule 83 including one or more HV output capacitors. When a shockablerhythm is detected, the HV capacitors are charged to a pre-programmedvoltage level by a HV charging circuit, which may include one or moretransformers, switches, diodes, or the like. Control module 80 applies asignal to trigger discharge of the HV capacitors upon detecting afeedback signal from therapy delivery module 84 that the HV capacitorshave reached the voltage required to deliver a programmed shock energy.In this way, control module 80 controls operation of the high voltageoutput circuit of therapy delivery module 84 to deliver high energyCV/DF shocks using defibrillation electrode 24 and housing 15. Highenergy CV/DF shocks are generally on the order of at least 5 Joules andmore commonly on the order of 20 Joules or higher. In contrast, lowvoltage pacing pulses delivered using extra-cardiovascular electrodesmay be on the order of 0.1 Joules or less, whereas pacing pulsesdelivered using endocardial electrodes or epicardial electrodes may beon the order of microJoules, e.g., 2 microJoules to 5 microJoules for atypical pacing pulse that is 2V in amplitude, 0.5 ms in pulse width andapplied across a pacing electrode vector impedance of 400 to 1,000 ohms.

Sensor module 92 may include additional sensors for monitoring thepatient and/or for controlling therapy delivery. For example, sensormodule 92 may include an activity sensor, a posture sensor, a heartsound sensor, or other physiological sensor(s) for monitoring thepatient and making therapy delivery decisions. In various examples, rateresponsive pacing may be provided based on a patient activity signal.The pacing rate delivered using extra-cardiovascular electrodes may beincreased according to an increased metabolic demand of the patient asevidenced by the patient activity signal. A decision to deliver ATPpulses (using extra-cardiovascular electrodes, e.g., electrodes 24, 28,30 and/or housing 15 or other extra-cardiovascular electrodeconfigurations referred to herein) or shock therapy (usingdefibrillation electrode 24 and housing 15) may be based in part onphysiological sensor signals in addition to the cardiac electricalsignal.

Control parameters utilized by control module 80 may be programmed intomemory 82 via telemetry module 88. For example, the pacing pulse widthand pacing pulse amplitude may be programmable parameters. Controlmodule 80 may utilize the programmed pacing pulse width and pacing pulseamplitude for controlling the selection and charging of LV capacitorsincluded in LV therapy delivery module 85. Telemetry module 88 includesa transceiver and antenna for communicating with external device 40(shown in FIG. 1) using RF communication as described above. Under thecontrol of control module 80, telemetry module 88 may receive downlinktelemetry from and send uplink telemetry to external device 40. In somecases, telemetry module 88 may be used to transmit and receivecommunication signals to/from another medical device implanted inpatient 12.

FIG. 4 is a schematic diagram of a pacing control module 102 included incontrol module 80 and the LV therapy delivery module 85 included intherapy delivery module 84. LV therapy delivery module 85 includes acapacitor selection and control module 104 and a capacitor array 110including multiple parallel capacitors C1 through Cn, where n is six inthe example shown. Capacitor array 110 includes multiple switches S1through Sn (S6 in the example shown) that are controlled by capacitorselection and control module 104 to selectively enable capacitors C1through C6 for pacing pulse delivery. Switches S1 through S6 each enablea respective one of capacitors C1 through C6 when closed by coupling theenabled capacitor to pacing pulse output signal line 130 when switch 112is also closed.

While six capacitors are shown, capacitor array 110 may include more orfewer capacitors, which may depend on the requirements of the particularpacing application, and available volume in the housing 15. CapacitorsC1 through C6 may be provided with a capacitance of 20 microfarads inone example but capacitances greater than or less than 20 microfaradsmay be used, e.g., 10 microfarads to 40 microfarads. Capacitors C1through C6 may all have the same capacitance values or different valuesto provide different selectable effective capacitances for achievingvarious RC time constants of the pacing discharge circuit and desiredranges of pacing pulse widths for an expected range of pacing electrodevector impedance.

A longer effective pulse width is possible when two or more ofcapacitors C1 through C6 are selected together in parallel than when anyone is selected alone. The capacitors in parallel have an effectivecapacitance equal to the sum of the parallel capacitances for a totalcapacitance greater than any one of the capacitors selected alone. Thegreater capacitance increases the RC time constant for a given pacingelectrode vector impedance. A higher capacitance and RC time constantdecreases the decay rate of the pacing pulse and increases the maximumpossible pacing pulse width for a given programmed pulse amplitude andpacing electrode vector impedance. The longer pulse width results ingreater pulse energy delivered across the pacing electrode vector forcapturing the heart. If the pulse decays too rapidly, the pulseamplitude at the expiration of the pulse width may be too low tosuccessfully capture the heart. In order to achieve capture using anon-painful, leading edge voltage amplitude of an extra-cardiovascularpacing pulse, a relatively long pulse width, e.g. greater than 2.0 ms,may be needed over which the decaying pulse amplitude is maintainedabove a minimum threshold so that the total delivered pulse energy isabove the capture threshold of the heart.

In one example, C1 has a higher capacitance than capacitors C2 throughC6 to provide a relatively long decay time when C1 is selected alone forpacing pulse delivery. C2 through C6 may be provided with lowercapacitance values than C1 so that capacitor configurations can beselected having higher effective capacitance values at desiredincrements greater than the C1 capacitance.

Capacitor selection and control module 104 controls LV charging circuit114 to charge capacitor array 110 for supplying pacing pulse energy. LVcharging circuit 114 charges capacitor array 110 to a voltage levelaccording to a programmed pacing pulse amplitude. Power source 98 mayprovide regulated power to LV charging circuit 114. LV charging circuit114 may be controlled to charge all capacitors C1-C6 or only selectedones of capacitors C1-C6 to a voltage required for generating a pacingpulse having a leading edge voltage amplitude at the programmed pulseamplitude. LV charging circuit 114 includes a charge pump to charge theparallel capacitors of array 110 when signaled by capacitor selectionand control module 104, which also closes switches S1 through S6 toenable charging of respective capacitors C1 through C6. LV chargingcircuit 114 may include a voltmeter or other indicator for providing afeedback signal to the charge pump during charging and a comparator todetermine when charging is complete, e.g., when the charge reaches theprogrammed pulse amplitude.

LV charging circuit 114 will monitor and control the charging ofcapacitors C1-C6 and may pass a control signal to capacitor selectionand control module 104 for controlling switch 112 to be open while LVcharging circuit 114 is charging to uncouple capacitor array 110 fromoutput signal line 130. LV charging circuit 114 may pass a chargecompletion signal to capacitor selection and control module 104 when thecapacitor selection is charged to a desired voltage. Switch 112 may beclosed by capacitor selection and control module 104 after chargecompletion and when it is time to start pacing pulse delivery.

Capacitor selection and control module 104 receives control signals andinstructions from pacing control module 102 including capacitorconfiguration data, pacing pulse timing data, and pacing pulse amplitudeand pulse width. In response to a signal from pacing control module 102,capacitor selection and control module 104 enables a selected capacitorconfiguration of capacitor array 110, by closing respective switches S1to S6, and couples the selected capacitor configuration to output signalline 130 via switch 112 to discharge the selected capacitorconfiguration across a pacing electrode vector coupled to output signalline 130. The capacitor selection and control module 104 uncouplescapacitor array 110 from output signal line 130 by opening switch 112 atthe expiration of a programmed pacing pulse width. The pacing pulse isterminated when switch 112 is opened.

In some examples, LV therapy delivery module 85 includes ananalog-to-digital converter (ADC) 106 for sampling the pacing pulseamplitude in real-time and providing a digital feedback signal of thesampled amplitude to pacing control module 102 on signal line 122.During the pacing pulse, pacing control module 102 enables ADC 106 tosample the pacing pulse output signal on output signal line 130 at adesired sampling rate, e.g., every 2 ms, throughout the pacing pulsewidth. ADC 106 may be enabled to sample the pacing pulse amplitude fromthe start of the pacing pulse when switch 112 is enabled (closed) untilthe end of the pacing pulse when switch 112 is disabled (opened). Inother examples, ADC 106 may be enabled at a predetermined time intervalafter the start of the pacing pulse, e.g. after a first portion of thepacing pulse width.

The ADC 106 does not disrupt the pacing pulse but samples the pulseamplitude and converts the analog pulse amplitude to a digitalrepresentation for processing by pacing control module 102. Pacingcontrol module 102 monitors the sampled pacing pulse amplitude receivedfrom ADC 106 during pacing pulse delivery by comparing the sample pointsto a pre-determined amplitude threshold or to an expected amplitudebased on predicted values. In one example, the sample points arecompared to an amplitude threshold set as a percentage of the programmedpacing pulse amplitude, e.g., 50% of the programmed pacing pulseamplitude.

If the pacing pulse amplitude falls below the amplitude threshold, thepacing control module 102 selects a second capacitor configurationhaving an appropriate capacitance and stored energy to compensate forthe decayed charge of the initial capacitor configuration. By enabling asecond capacitor configuration that is holding the charged voltageamplitude, the pacing pulse amplitude is increased during pulsedelivery, preserving the effective longevity of the pacing pulse widthand delivered energy required for capturing of the heart. In this way,the capacitor selection can be reconfigured during a single pacing pulseto increase the pacing pulse voltage amplitude in the event of a fasterthan anticipated decay rate. The capacitor selection and control module104 is capable of managing the switches S1 to S6 to achieve real-timecapacitor reconfiguration during a pacing pulse. The sampling interval,e.g., 2 ms or less, may be adjusted as needed to enable real-timeadjustment of the capacitor configuration from an initial capacitorconfiguration to a second capacitor configuration during the pacingpulse to maintain the pulse amplitude above a minimum acceptablethreshold.

In one example, the second capacitor configuration is selected bypassing a control signal to capacitor selection and control module 104to enable at least one capacitor of capacitor array 110 that was notincluded in the initial capacitor configuration. The capacitor(s) of thesecond capacitor configuration restore a higher pacing pulse amplitudeto ensure the programmed pulse width and required total pulse energy tocapture the heart is achieved, even under changing in vivo conditionssuch as changing impedance along the pacing vector.

All or at least a portion of capacitors C1 through C6 are charged priorto pacing pulse delivery. At least one capacitor more than the number ofcapacitors being selected in the initial capacitor configuration may becharged prior to pacing pulse delivery. In response to a control signalfrom pacing control module 102, capacitor selection and control module104 reconfigures the initial capacitor configuration to a secondcapacitor configuration that includes at least one capacitor not used inthe initial capacitor configuration.

In an illustrative example, C1, C2 and C3 may be initially enabled bycapacitor selection and control module 104. Capacitors C1, C2, and C3are collectively discharged across the pacing electrode vector viaoutput signal line 130 to begin delivery of the pacing pulse at thedesired pulse amplitude. The output of capacitors C1, C2, and C3 decaysover time during capacitor discharge. If pacing control module 102determines that the sampled pulse amplitude has reached or fallen belowa pre-determined threshold during the pacing pulse, pacing controlmodule 102 passes a signal to capacitor selection and control module 104to couple at least one new capacitor to pacing pulse output signal line130. For example, capacitor selection and control module 104 may enablecapacitor C4 via switch S4. By adding a capacitor C4 to thepreviously-enabled, discharging capacitors C1, C2 and C3 during pacingpulse delivery, the pulse amplitude may be maintained above a minimumpulse amplitude throughout the pacing pulse width. C1, C2 and C3 may bedisabled or switched off when C4 is enabled to ensure all the energyfrom C4 is directed to the output signal line 130 via switch 112. Inother examples, circuitry such as one or more diodes may be included incapacitor array 110 to prevent charge distribution from newly-enabledcapacitor(s) to the partially-discharged initial capacitor(s) andpromote current flow produced by all discharging capacitors to outputline 130 so that all capacitors discharge across the pacing electrodevector.

The added capacitor C4 in the second capacitor configuration may bepreviously charged, prior to pacing pulse delivery. Frequent sampling ofthe pulse amplitude may allow an amplitude threshold crossing to bepredicted and thereby allow charging of additional capacitors on an asneeded basis. Pacing control module 102 may trigger capacitor selectionand control module 104 to charge an additional capacitor(s), C4 in thisexample, in preparation for enabling a second capacitor configuration ifneeded.

Capacitor selection and control module 104 may control which ofcapacitors C1 through C6 are re-charged after delivering a pacing pulse.In some examples, all available capacitors C1 through C6 are fullycharged between pacing pulses and remain charged if not used for pacingpulse delivery. The charge of any unused capacitors may be topped offbetween pacing pulses. In other cases, the number of capacitors beyondthe initial capacitor configuration that are charged and available foradding to the discharge circuit during pacing pulse delivery may includeall the remaining available capacitors or a preset number of additionalcapacitors, e.g., one to three additional capacitors. In other examplesthe number of additional capacitors may depend on the pulse width. Toillustrate, for very long pulse widths, e.g., greater than 10 ms, allavailable capacitors may be charged. For relatively short pulse widths,e.g., less than 5 ms, one additional capacitor may be charged. Formoderate pulse widths, e.g., from 5 ms to 10 ms, two additionalcapacitors may be charged. Since the pulse amplitude may be more likelyto fall below a minimum threshold amplitude before the end of the pacingpulse when longer pulse widths are used, a greater number of additionalcapacitors may be charged to be available during pacing pulse delivery.The number of additional capacitors available may be limited by how manyare being used in the initial capacitor configuration.

FIG. 5 is a schematic diagram of pacing control module 102 included incontrol module 80 and capable of accessing instructions stored in memory82. Pacing control module 102 may include a microprocessor 140, databuffer 144, capacitor configuration module 146, ADC control module 148and timing control module 150. Microprocessor 140 may be configured toexecute instructions stored in memory 82 for selecting an initialcapacitor configuration for delivering a pacing pulse and forautomatically adjusting the capacitor configuration during the pacingpulse.

Microprocessor 140 provides capacitor configuration data to capacitorconfiguration module 146 which passes the capacitor configuration datato the capacitor selection and control module 104 of LV therapy deliverymodule 85 (FIG. 4). Microprocessor 140 may also pass instructions to ADCcontrol module 148. ADC control module 148 may be configured to controlthe sampling rate and sampling time period(s) of ADC 106 (FIG. 4). Pulseamplitude sample points are received by data buffer 144 from ADC 106 andpassed to microprocessor 140. Microprocessor 140 compares the sampledamplitude values to an amplitude threshold. Based on this comparison,microprocessor 140 determines if a capacitor configuration change isrequired. If the sampled pulse amplitude is at or below the threshold, acapacitor configuration adjustment is needed to maintain the pulseamplitude above a minimum acceptable amplitude throughout the programmedpulse width.

Microprocessor 140 passes new configuration data to capacitorconfiguration module 146 in response to the sampled amplitude falling toor below the threshold amplitude. Capacitor configuration module 146 inturn passes the new configuration data to capacitor selection andcontrol module 104 of LV therapy delivery module 85, e.g., on the nextclock signal. The capacitor configuration is now set until anotherreconfiguration occurs. In this way, the pacing pulse output signalamplitude is adjusted by changing the capacitor configuration duringdelivery of the pacing pulse, i.e., before the pacing pulse widthexpires, in real-time.

Data buffer 144 may receive impedance data from impedance measurementmodule 90. Microprocessor 140 may retrieve the impedance measurementdata for use in determining an initial capacitor configuration and/or inselecting an adjusted capacitor configuration in response to the sampledpulse amplitude falling to or below a threshold. A request for impedancedata may be made after an amplitude threshold crossing during pulsedelivery to assist in determining a capacitance required in the secondcapacitor configuration to adjust the pacing pulse amplitude andmaintain it above a minimum amplitude. A new impedance measurementrequest may additionally or alternatively be made between pacing pulsesbased on the pulse amplitude reaching an amplitude threshold during theprevious pacing pulse.

Microprocessor 140 may determine the initial capacitor configuration bycomputing the capacitance required to achieve an RC time constant of thecapacitor array and known impedance measured for the selected pacingvector that enables maintaining the pulse amplitude above a minimumamplitude for at least the programmed pacing pulse width. Operationsperformed for determining a capacitor configuration are described belowin conjunction with FIGS. 9 through 11.

Timing control module 150 receives pacing pulse timing data frommicroprocessor 140, including starting time of a scheduled pacing pulseand the pulse width. The time to start pacing pulse delivery and thepulse width are passed to capacitor selection and control module 104.Capacitor selection and control module 104 selects the capacitorconfiguration according to the configuration data received fromcapacitor configuration module 146 and couples the selected capacitorconfiguration to the output signal line 130 according to a pacing pulsestart time passed from timing control module 150, e.g., based on theexpiration of a pacing escape interval or other inter-pulse interval.Capacitor selection and control module 104 uncouples the selectedcapacitor configuration from the output signal line 130 upon expirationof the pacing pulse width received from timing control module 150.

FIG. 6 is a schematic diagram of capacitor selection and control module104 according to one example. Capacitor selection and control module 104includes a configuration control module 160, configuration latches 162,a pulse width timer 164 and pacing pulse enable/disable gate 166.Configuration control module 160 receives a clock signal 154 and aninput signal 156 from capacitor configuration control module 146 ofpacing control module 102 (FIG. 5). The input signal 156 includescapacitor configuration data indicating the number of capacitors C1through C6 that are to be coupled to output signal line 130 fordelivering the next pacing pulse.

Configuration control module 160 clocks the capacitor configuration datastored in buffers or other memory devices to configuration latches 162,which store the configuration data until passed to the S1-S6 switches ofthe capacitor array 110 (FIG. 4). In accordance with the configurationdata, configuration latches 162 set separate signals that are passed toeach of the respective switches S1-S6 to selectively enable or disableeach one of capacitors C1 through C6 for pacing pulse delivery. Disabledcapacitors may be charged and remain charged until needed for pacingpulse delivery but are not coupled to output signal line 130 for pacingpulse delivery. Disabled capacitors may be initially inactive during apacing pulse, i.e., not coupled to output signal line 130, but are readyto be enabled for pacing pulse delivery if an adjusted capacitorconfiguration is deemed necessary during the pacing pulse. Disabledcapacitors are enabled by being coupled to output signal line 130 viaappropriate control of switches S1 through S6.

Configuration control module 160 receives initial capacitorconfiguration data via input signal 156 that sets the configuration datafor selectively enabling or disabling each one of capacitors C1 throughC6 according to an initial capacitance requirement to achieve an RC timeconstant that is longer than the pacing pulse width or other predefinedtime interval threshold. The configuration data is passed toconfiguration latches 162 which enable the capacitor(s) included in theinitial capacitor configuration to be used for pacing pulse delivery.

Pulse width timer 164 receives clock signal 154 and input from timingcontrol module 150 (FIG. 5) on signal line 158. Pulse width timer passesa timing control signal to pulse enable/disable gate 166. For example,upon expiration of a pacing escape interval, timing control module 150passes a signal to pulse width timer 164 to enable LV therapy deliverymodule 85 to start a pacing pulse. Pulse enable/disable signal gate 166outputs a signal on signal line 168 to switch 112 (FIG. 4) to start thepacing pulse. Switch 112 is controlled by gate 166 to couple theselected capacitor configuration to pacing pulse output signal line 130.

After the initial capacitor configuration is coupled to the pacing pulseoutput signal line 130 via switch 112, configuration control module 160may receive new capacitor configuration data via signal line 156 frompacing control module 102 if the sampled pacing pulse amplitude falls toor below an amplitude threshold. As described above, pacing controlmodule 102 receives the sampled pacing pulse output signal amplitude andcompares the sampled amplitude to the amplitude threshold. If the pacingpulse amplitude does not fall to or below an amplitude threshold duringthe pulse width, the initial capacitor configuration remains unchangedduring the pacing pulse. If the amplitude falls to or below theamplitude threshold during the pacing pulse width, new capacitorconfiguration data is passed to configuration control module 160 duringthe present pacing pulse delivery. Configuration control module 160passes new configuration data to configuration latches 162 which causesat least one capacitor to be enabled that was not included in theinitial capacitor configuration by coupling the capacitor in with theinitial capacitor configuration via a respective switch (one of switchesS1-S6). Capacitors selected for the initial configuration may bedeactivated (switched out of the discharge circuit by uncoupling fromoutput signal line 130) in the adjusted capacitor configuration toprevent charge distribution to those capacitors rather than to the pulseoutput signal line 130 in some cases. New configuration data may bepassed to configuration control module 160 and onto configurationlatches 162 multiple times during a given pacing pulse if the sampledpacing pulse amplitude decays to or below an amplitude threshold morethan once before the pulse width expires due to the programmed pulseduration exceeding the discharge capacity of the currently selectedcapacitor configuration.

Upon expiration of the pacing pulse width, pulse width timer 164 passesa pulse termination signal to pulse enable/disable gate 166 that outputsa signal on control signal line 168 that terminates the pacing pulse bydisabling switch 112 to uncouple the selected capacitor configurationfrom output signal line 130. Pacing control module 102 passes capacitorconfiguration data to configuration control module 160 for the nextscheduled pacing pulse. The capacitor configuration for the nextscheduled pacing pulse may be re-determined by microprocessor 140 basedon a new pacing vector impedance measurement or selected as the finalcapacitor configuration upon expiration of the preceding pacing pulse.Prior pacing pulse amplitude behavior, which may be influenced by pacingvector impedance variations, may influence the capacitor configurationselected for the next pacing pulse delivery.

FIG. 7A is a conceptual diagram of a pacing pulse 200 generated by LVtherapy delivery module 85. Pacing pulse 200 has an initial or leadingedge pulse amplitude 202 determined by the charge value of the capacitorconfiguration, in accordance with the programmed pacing pulse voltageamplitude. The leading edge pulse amplitude may be, for example with nolimitation intended, approximately 5 V to 8 V. The pacing pulse leadingedge 206 occurs when pulse enable/disable gate 166 enables or closesswitch 112 to electrically couple a selected capacitor configuration ofcapacitor array 110 to output signal line 130. The selected capacitorconfiguration discharges through the impedance of the pacing electrodevector during the predetermined pacing pulse width 210. Upon expirationof the pacing pulse width 210, the pulse enable/disable gate 166disables or opens switch 112 to terminate the pacing pulse atterminating edge 208. The pacing pulse has a truncated pulse amplitude204 at the expiration of the pacing pulse width 210.

The amplitude of the pacing pulse 200 decays from the leading edgeamplitude 202 to the truncated amplitude 204 over the pacing pulse width210 as the selected capacitor configuration discharges through thepacing electrode vector impedance according to an RC time constant 212.The RC time constant 212, sometimes referred to as “tau,” is the productof the pacing electrode vector impedance and the capacitance of theselected capacitor configuration. By definition, the RC time constant isthe time for a capacitor to discharge through a resistance toapproximately 36.8% of its initial charge. A change in pacing vectorimpedance alters the impedance in the RC time constant, altering thedischarge rate of the capacitor configuration, observed as the decayrate of the pacing pulse amplitude. In some examples, the pacing controlmodule 102 is configured to detect a decay rate that is occurring tooquickly for the pacing pulse width 210 by comparing sampled pulseamplitude values to a threshold amplitude and responding by adjustingthe capacitor configuration to restore a higher pulse amplitude, aslower decay rate or both.

Microprocessor 140 of pacing control module 102 is configured todetermine a capacitor configuration including one or more capacitors ofcapacitor array 110 having an overall capacitance that, with themeasured impedance of the pacing electrode vector, results in an RC timeconstant 212 that is greater than the programmed pacing pulse width 210.In some examples, the capacitor configuration is selected such that thetruncated amplitude 204 is expected to be greater than a predeterminedpercentage of the leading edge amplitude 202, e.g., greater than 50% ofthe leading edge amplitude 202. A threshold voltage requirement of thetruncated amplitude 204 may be defined in order to promote successfulcapture of the myocardium prior to or upon termination of the pacingpulse 200. When extra-cardiovascular electrodes are used for deliveringpacing pulses, the leading edge amplitude 202 may be kept relativelylow, e.g., 8 Volts or less and below a pain threshold of the patient. Inorder to deliver enough energy to successfully capture the myocardium, arelatively long pacing pulse width 210 may be required. If the RC timeconstant 212 of a selected capacitor configuration is too short, thepacing pulse amplitude may decay too quickly such that the truncatedamplitude 204 is below a minimum threshold and the total pacing pulseenergy is inadequate to capture and pace the heart.

In order to promote successful capture, the pacing control module 102may be configured to select the capacitor configuration so that the RCtime constant 212 is greater than a time interval threshold. Forexample, the capacitor configuration may be selected to have acapacitance resulting in an RC time constant 212 that is at leastgreater than the programmed pacing pulse width 210 or a multiple thereofor greater than a maximum programmable pacing pulse width or a multiplethereof, e.g., twice or three times the maximum programmable pacingpulse width. In one example, the pulse width may be programmable between1.5 ms and 20 ms. The capacitor configuration is selected based on themeasured impedance of the pacing vector so that the resulting RC timeconstant 212 of the discharge circuit (the selected capacitorconfiguration discharging through the pacing electrode vector impedance)is longer than the pacing pulse width 210 and the truncated amplitude204 is greater than an amplitude threshold at the expiration of thepacing pulse 200. By selecting a capacitor configuration having acapacitance that results in an RC time constant meeting these criteria,the voltage amplitude over the duration of the pacing pulse iscontrolled within an acceptable voltage range and the delivered pulseenergy can be expected to successfully pace the heart.

FIG. 7B is a conceptual diagram of a pacing pulse 220 that may bedelivered by IMD 14 using a capacitor configuration that is adjustedduring the pacing pulse delivery. Pacing pulse 220 has an initial orleading edge pulse amplitude 222 determined by the charge value of thecapacitor configuration, in accordance with the programmed pacing pulsevoltage amplitude. The pacing pulse leading edge 226 occurs when pulseenable/disable gate 166 enables or closes switch 112 to electricallycouple a selected capacitor configuration of capacitor array 110 tooutput signal line 130. The selected capacitor configuration dischargesthrough the impedance of the pacing electrode vector with a decay rate223 that may be faster than expected causing the pacing pulse amplitudeto fall below an amplitude threshold 225 prior to expiration of theprogrammed pacing pulse width 210.

Pacing control module 102 may monitor the sampled pulse amplitude, and,if the pulse amplitude falls to the threshold 225, pacing control module102 passes a new capacitor configuration to capacitor selection andcontrol module 104. The adjusted capacitor configuration is enabled at227. Upon enabling the adjusted capacitor configuration, the pulseamplitude is increased above the threshold 225 and decays at a seconddecay rate 229 until the pulse width 210 expires. The second decay rate229 may be the same or different than the first decay rate 223 dependingon the relative capacitances of the initial capacitor configuration andthe adjusted capacitor configuration. The increase in pulse amplitudeachieved by coupling at least one charged capacitor to the dischargecircuit during the pacing pulse 220 maintains the pacing pulse amplitudewithin an acceptable voltage range, e.g., between and including theprogrammed pulse amplitude (corresponding to leading edge amplitude 222)and the amplitude threshold 225. The adjusted capacitor configurationprevents the pulse amplitude from falling below the amplitude threshold225. The truncation amplitude 224 is greater than the amplitudethreshold 225 at terminating edge 228 of pulse 220.

In some examples, threshold 225 is set greater than a minimum acceptablepulse amplitude threshold so that the capacitor configuration can beadjusted before the pulse amplitude reaches the minimum acceptablethreshold. For example, if the programmed pulse amplitude is 5 V, aminimum acceptable threshold may be 2.5 V. The amplitude threshold 225used for triggering a capacitor configuration adjustment may be 2.75 Vso that the pulse amplitude is always maintained above the minimumacceptable threshold.

Extra-cardiovascular leads and pacing electrodes may be subjected tobody motion resulting in shifting of the pacing electrodes and changesin the pacing electrode vector impedance. Acute or chronic changes inthe impedance of a pacing electrode vector will be accounted for indetermining a capacitor configuration for a given measured impedance. Insome cases, pacing electrode vector impedance may change during a pacingpulse. When pacing control module 102 is configured to monitor the pulseamplitude in real time, the pacing control module 102 can respond to achange in pacing electrode vector impedance during a pacing pulse byadjusting the capacitor configuration.

FIG. 7C is a conceptual diagram of one example of a pacing pulse 230that may be delivered by IMD 14 using an adjusted capacitorconfiguration during the pacing pulse when a pacing electrode vectorimpedance change occurs. Pacing pulse 230 has a pulse amplitude 232 atthe leading edge 236 that is determined by the charge value of theinitial capacitor configuration that is selected and based on theprogrammed pacing pulse amplitude. The pacing pulse decays from theleading edge amplitude 232 at an initial decay rate 233 a according tothe RC time constant of the discharge circuit. The RC time constant is afunction of the capacitance of the initial capacitor configuration andthe impedance of the pacing electrode vector. At time 231, during thepacing pulse 230, a change in pacing vector impedance occurs, e.g., dueto a shift in electrode location or other factors. A decrease inimpedance causes the pulse to decay at a faster decay rate 233 b aftertime 231 in this example. The faster decay rate 233 b causes the pulseamplitude to reach amplitude threshold 235 prior to the expiration ofpulse width 210.

In this example, pacing control module 102 is configured to monitor thepulse amplitude samples received from ADC 106 in real-time and determineif the pulse amplitude reaches the amplitude threshold 235 during thepacing pulse. As described previously, the threshold 235 may be set as apercentage of programmed pulse amplitude and may define a minimumacceptable voltage amplitude or be greater than the minimum acceptablevoltage amplitude so that the pulse amplitude can be adjusted in realtime to be maintained within an acceptable voltage range.

Upon detecting that the pulse amplitude has reached the amplitudethreshold 235, pacing control module 102 passes adjusted capacitorconfiguration data to capacitor selection and control module 104.Capacitor selection and control module 104 enables the new capacitorconfiguration to begin discharging across the pacing vector resulting ina step increase 237 of the pacing pulse amplitude. The step increase 237is achieved by switching in at least one fully charged capacitor tobegin discharging during the pacing pulse 230. Capacitors included inthe initial capacitor configuration may be disabled or may remainenabled in the adjusted capacitor configuration.

The pulse 230 decays at a third decay rate 239 after the step increasein amplitude. The third decay rate 239 will depend on the capacitance ofthe adjusted capacitor configuration and the impedance of the pacingelectrode vector. In this example, the capacitance of the adjustedcapacitor configuration may be the same as the initial capacitorconfiguration such that decay rate 239 is approximately the same as thesecond decay rate 233 b after the pacing vector impedance change at 231.The step increase 237 in pulse amplitude, however, maintains the pulseamplitude within an acceptable range such that the truncated amplitude234 at terminating edge 238 is still greater than the amplitudethreshold 235. In other examples, the adjusted capacitor configurationmay be selected to have a higher capacitance than the initial capacitorconfiguration to slow the decay rate 239 after the decrease in pacingelectrode vector impedance.

FIG. 8 is a conceptual diagram of a look-up table 240 stored in memory82 accessible by pacing control module 102. Various capacitorconfigurations may be stored in memory 82, e.g., in the form of alook-up table, for different values of pacing electrode vectorimpedance. The look-up table 240 includes multiple impedance ranges 242(listed in ohms), and a capacitor configuration 244 stored for eachimpedance range. Microprocessor 140 may be configured to fetch animpedance measurement from data buffer 144 (FIG. 5) and compare theimpedance measurement to the multiple impedance ranges 242 included inthe look-up table 240. When the range of impedance is identified thatincludes or matches the impedance measurement, the microprocessor 140selects the capacitor configuration that is stored for the matchingimpedance range.

As seen in the example of FIG. 8, the capacitor configurations 244 rangefrom a capacitor configuration including all six capacitors, C1-C6, ofcapacitor array 110 when the measured impedance is between 100 and 249ohms, inclusive, to a capacitor configuration including only onecapacitor, C1, when the measured impedance is between 1600 and 2000ohms, inclusive.

Using the example of capacitors C1-C6 each being 20 microfaradcapacitors, the RC time constant for the first capacitor configurationC1-C6 is approximately 12 ms to 30 ms, depending on the measuredimpedance in the first range of 100 ohms to 240 ohms. The RC timeconstant is between approximately 25 ms and 50 ms for the secondcapacitor configuration C1-C5, between approximately 40 and 75 ms forthe C1-C4 configuration, between approximately 45 and 72 ms for theC1-C3 configuration, between approximately 48 and 64 ms for the C1-C2configuration and between approximately 32 and 40 ms for the C1configuration. Accordingly, for a pulse width of 10 ms, the RC timeconstant for a given impedance measurement and its associated capacitorconfiguration is greater than the 10 ms pulse width for all possiblecapacitor configurations. If the maximum programmable pulse width is ashigh as 20 ms, all capacitor configurations have an RC time constantgreater than the maximum programmable pulse width with the exception ofthe case when the pacing vector impedance approaches the minimum listedimpedance of 100 ohms.

A pacing vector impedance of 100 ohms is relatively low, but may occurin some instances. For pacing pulse widths of 10 ms or greater, theleading edge pulse amplitude may be required to be relatively higher inorder to deliver adequate energy prior to expiration of the pacingpulse. For a shorter pulse width, e.g., 5 ms, the truncated amplitude is3.29 V when the leading edge amplitude is 5 V and the capacitorconfiguration C1-C6 is used. The truncated amplitude is greater than 50%of the leading edge amplitude.

In other examples, a stored look-up table such as table 240 is notrequired. Microprocessor 140 may compute the capacitance required toobtain an RC time constant greater than a threshold time interval forthe known, measured impedance. Microprocessor 140 may then select thenumber of capacitors C1-C6 required to achieve an overall capacitancethat is equal to or greater than the computed, required capacitance.

In the example look-up table 240 of FIG. 8, C1 is the default capacitorenabled for all pacing pulses. Capacitors C2 through C6 may be added toa capacitor configuration as needed in a Cn+1 order, e.g., theconfiguration of C1-C2 may be selected if two capacitors are needed, theconfiguration C1-C2-C3 may be selected if three capacitors are neededand so on up to C1 through C6 if all capacitors are included in theselected capacitor configuration.

In other examples, when additional capacitors are needed, capacitorselection and control module 104 may cycle through the remainingcapacitors from one pacing pulse to the next. For example, C1 may remainthe default capacitor but be paired with a different second capacitorfor successive pacing pulses when two capacitors are needed, e.g., C1and C2 for one pacing pulse, then C1 and C3 for the next pacing pulse,then C1 and C4, and so on. In other examples, capacitor selection andcontrol module 104 may cycle the capacitors selected for generatingpacing pulses for a given capacitance requirement of the capacitorconfiguration. For example, if the selected capacitor configurationincludes three capacitors, capacitors C1, C2 and C3 may be enabled fordelivering one pacing pulse and capacitors C4, C5 and C6 may be used fordelivering the next pacing pulse such that the capacitors are chargedand enabled for pulse delivery in an alternating manner. It is to beunderstood that practice of the techniques disclosed herein are notlimited to a particular order or sequence of selecting capacitorsincluded in a capacitor configuration. Rather, the capacitors of array110 may be selected in any combination in order to achieve a requiredcapacitance to achieve a pacing pulse having the programmed pulse widthand truncated voltage amplitude that successfully captures the heartwith a high degree of confidence. By cycling or alternating betweencapacitors of array 110 when not all capacitors are required may allowthe LV therapy module 104 to maintain charge on capacitors betweenpacing pulses, particularly when the pacing rate is relatively fast(shorter escape interval), such as during ATP or rate responsive pacingat a higher rate. If capacitors are charged between pulses to maintain acharge, topping off pacing charge can be performed quickly. If morecomplete charging is needed, cycling between capacitor configurationsallows one capacitor configuration to be charged for every other pacingpulse while a different capacitor configuration is delivering theintervening pacing pulses.

FIG. 9 is a flow chart of a method performed by IMD 14 for delivering apacing pulse according to one example. At block 252, the control module80 controls impedance measurement module 90 to measure the impedance ofa selected pacing electrode vector, e.g. from electrode 28 to housing15, from electrode 30 to housing 15, or between electrodes 28 and 30 ofFIGS. 1 and 2. At block 254, microprocessor 140 of pacing control module102 receives the impedance measurement and determines a capacitorconfiguration for delivering a pacing pulse across the measuredimpedance of the selected pacing vector for the desired pulse width.

The capacitor configuration may be determined from a look-up tablestored in memory 82 as described in conjunction with FIG. 8.Alternatively, the capacitor configuration may be determined bycomputing the total capacitance required to achieve an RC time constantthat is greater than a threshold time interval for the measuredimpedance of the selected pacing vector. The threshold time interval maybe based on the programmed pulse width and/or a minimum amplitude at theterminating pulse edge. The capacitor configuration is then selected atblock 256 as the number of capacitors required to meet or exceed therequired total capacitance and by identifying the capacitors ofcapacitor array 110 that are to be enabled in the initial capacitorconfiguration.

In some examples, the capacitor configuration is determined to achievean RC time constant that is greater than the delivered pulse width sothat the truncated pulse amplitude at the expiration of the deliveredpulse width is greater than a minimum threshold. The minimum thresholdat the expiration of the pulse width may be a predetermined percentageof the leading edge pulse amplitude, e.g., 50% of the leading edge pulseamplitude.

To illustrate, if the pacing pulse width is programmed to 10 ms, themeasured impedance of the pacing electrode vector is 500 ohms, and thecapacitor array 110 includes six 20 microfarad capacitors, the capacitorconfiguration may be determined at block 254 to include three capacitorshaving a combined capacitance of 60 microfarads. The resulting timeconstant is 30 ms, greater than the programmed pulse width of 10 ms. Ifthe pacing pulse amplitude is programmed to 5.0 V at the leading edge ofthe pacing pulse, the truncated voltage amplitude at the expiration ofthe 10 ms pulse width is expected to be approximately 3.58 V, greaterthan at least 50% of the leading edge voltage amplitude.

At block 256, the capacitor selection and control module 104 enables theselected capacitor configuration, e.g., by enabling or disablingswitches S1 through S6 as required to enable the capacitors selectedfrom C1 through C6 of capacitor array 110. The selected capacitors arecharged at block 258 and the pulse width timer 164 is set at block 260.The pacing pulse is delivered at block 262 by enabling switch 112 tocouple the selected capacitor configuration to output signal line 130 toallow the capacitor configuration to discharge across the pacingelectrode vector. When the pulse width timer 164 expires, switch 112 isdisabled or opened to terminate the pacing pulse and uncouple thecapacitor configuration from output signal line 130 at block 264.

In flow chart 250, capacitor charging is performed after selection ofthe individual capacitors of capacitor array 110 for use in thedetermined capacitor configuration. In some examples, only thecapacitors included in the initial capacitor configuration are chargedfor pacing pulse delivery. In other examples, at least one additionalcapacitor is charged to be available to be added to the initialcapacitor configuration if the pacing pulse amplitude falls to or belowa threshold. In still other examples, all capacitors of capacitor array110 are charged prior to, during or after determination of the capacitorconfiguration to be used for the next pacing pulse. Only the capacitorsselected according to the initial capacitor configuration are coupled tothe output signal line 130 for initiating the pacing pulse. Anycapacitors not included in the initial capacitor configuration mayremain charged and available for selection in a second capacitorconfiguration selected in response to the pulse amplitude falling belowan amplitude threshold prior to pulse width expiration or for selectionin an initial capacitor configuration for delivering a future pacingpulse.

After pacing pulse delivery, the process shown in FIG. 9 returns toblock 252 to repeat the impedance measurement for the pacing electrodevector. In some examples, the impedance measurement is performed priorto each pacing pulse so that the initial capacitor configuration is setin response to an impedance measurement for each pacing pulse. In otherexamples, after setting the initial capacitor configuration for thefirst pulse after an impedance measurement, the same capacitorconfiguration may be used for subsequent pacing pulses until a capacitorconfiguration adjustment is required or until another impedancemeasurement occurs, e.g., on a scheduled basis.

FIG. 10 is a flow chart 270 that includes operations that may beperformed in block 262 of FIG. 9 for delivering a pacing pulse. Suchoperations include pacing pulse initiation and termination and amplitudesampling during pulse delivery. The method of flow chart 270 may alsoinclude post-pulse delivery assessment of the capacitor configurationused to deliver the pulse to guide selection of the next initialcapacitor configuration for delivering the next pacing pulse. At block271, the pacing pulse is initiated by coupling the selected capacitorconfiguration to the output signal line 130 at the appropriate time ascontrolled by pulse width timer 164. The pacing pulse has a leading edgevoltage amplitude in accordance with the programmed pulse amplitude.

At block 272, the pacing control module 102 enables the ADC 106 to beginsampling the pacing pulse amplitude at a predetermined sampling rate,e.g., 2 milliseconds. Sampling occurs concurrently with pulse deliveryto provide real-time detection of an amplitude threshold crossing. Inone example, the ADC 106 is enabled at the leading edge of the pacingpulse and the pacing pulse amplitude is sampled throughout the pacingpulse from the leading edge or the first sampling interval thereafteruntil the terminating edge or at the last sampling interval precedingthe terminating edge. In other examples, the ADC 106 may be enabled tobegin sampling after a time delay after the leading edge, e.g. after aportion of the pulse width. For instance, ADC 106 may be enabled tobegin sampling half-way through the pacing pulse width and continuesampling the pacing pulse amplitude until the terminating edge or thelast sampling interval preceding the terminating edge. In still otherexamples, the sampling interval may be variable. A longer samplinginterval may be used initially and shorten as the terminating edge isapproached.

In some cases, the pulse amplitude may be sampled at a mid-point of thepacing pulse width and if the pulse amplitude is at least an expectedamplitude or greater, no further sampling is performed during the pacingpulse. Using the example given above, for a 5V pulse having a 10 mspulse width that is expected to have a truncated amplitude of 3.58 V at10 ms, if the amplitude is sampled at 5 ms and is at least an expectedamplitude based on predictive modeling, e.g., 4.2 V, no further samplingis needed. A reasonable confidence exists that the truncated amplitudewill remain above the targeted minimum voltage, e.g., 50% of the leadingedge voltage or 2.5 V in this example. If the sampled amplitude is lessthan an expected midpoint amplitude, however, ADC 106 may be enabled tosample the pacing pulse at a predetermined sampling rate, e.g., every 2ms or more often, for the remainder of the pacing pulse width.

At block 274 the microprocessor 140 fetches the sampled signal pointfrom data buffer 144 and compares the sampled signal point to anamplitude threshold at block 276. The amplitude threshold used for thecomparison at block 276 may be defined as a minimum threshold, e.g., 50%or another percentage of the leading edge amplitude (or programmed pulseamplitude). As suggested above, the amplitude threshold may be a higherthreshold used for comparison to sample points acquired earlier in thepacing pulse width. For example, at the midpoint of the pacing pulsewidth, the sample point may be compared to a higher threshold, e.g., 85%of the programmed pulse amplitude. From the midpoint to the terminatingedge of the pulse width, the sampled amplitude may be compared to alower threshold, e.g., 50% of the programmed pulse amplitude. In otherexamples, the threshold may be a function of the expired time of thepacing pulse width and based on an expected amplitude according to theRC time constant of the capacitor configuration and measured impedanceof the pacing vector.

If the sampled pulse amplitude is less than the amplitude threshold atblock 276, the capacitor configuration may be adjusted at block 280. Asdescribed above, in some cases, if the pulse amplitude is below a firstthreshold during a first portion of the pacing pulse width, the ADC 106may be enabled to sample more frequently during a second portion of thepacing pulse width during which a second lower threshold is used for thecomparison at block 276. When the sampled amplitude falls below thesecond lower threshold, the capacitor configuration adjustment may bemade at block 280. In other examples, the capacitor configurationadjustment may be made in response to the sampled amplitude falling toor below the first higher threshold during the first portion of thepacing pulse if detected first and otherwise in response to the sampledamplitude falling to or below the second lower threshold during thesecond portion of the pacing pulse.

In some examples, a limited number of capacitor configurationadjustments may be performed. For instance, after the capacitorconfiguration has been adjusted the first time during a pacing pulse atblock 280, no further adjustments of the capacitor configuration aremade. The ADC 106 may or may not continue to sample the pulse amplitudeduring the remainder of the pacing pulse after the pacing configurationhas been changed. A maximum number of capacitor configurationadjustments greater than one may be allowed, e.g., up to two, three ormore adjustments, in which case the ADC 106 continues to sample thepulse amplitude for comparison to the amplitude threshold at block 276after the first adjustment. If the maximum number of adjustments hasbeen reached as determined at block 277, the process may advance toblock 278 to wait for the pulse width timer 164 to expire. If themaximum number of capacitor configuration adjustments has not beenreached at block 277, and the pulse width timer has not expired (block279), the capacitor configuration is adjusted at block 280.

After adjusting the capacitor configuration at block 280, ADC 106 maycontinue sampling the pacing pulse signal at block 274 and passingsignal sample points to data buffer 144 which may store the data on afirst-in, first-out basis. If the pacing pulse amplitude falls below athreshold again before the pulse width timer expires, the capacitorconfiguration may be adjusted again prior to termination of the pacingpulse if the maximum number of adjustments has not been reached.

The ADC 106 may be controlled to sample the pulse amplitude at a regularsampling interval from the onset of the pulse or a predetermined timeinterval after onset of the pulse until expiration of the pulse width.In other examples, the ADC 106 may be controlled to be disabled prior tocompletion of the pulse delivery if the pulse amplitude has beensustained at an acceptable level up to a predetermined portion of thepulse width. The time that the ADC 106 is disabled may be based onsampling rate, the programmed pulse width, the expired portion of thepulse width and the pulse amplitude stability during the pulse. “Pulseamplitude stability” refers to the amplitude remaining within a“constant” voltage range as defined for a particular application. Thepulse amplitude may decay over the pulse width but remain within aspecified “constant” voltage range, e.g., between amplitude thresholdand the programmed pulse amplitude. If not, pulse amplitude samplingcontinues and the capacitor configuration is adjusted as needed. In someexamples, all or some of the sample points obtained during the pacingpulse may be passed to memory 82 to be stored for transmission toexternal device 40 for reviewing and analyzing pacing performance of IMD14.

If ADC 106 continues to sample the pulse amplitude after the capacitorconfiguration adjustment while waiting for the pulse width to expire,the additional amplitude data may be used by microprocessor 140 forselecting future initial capacitor configurations for subsequent pacingpulses and/or for selecting adjusted capacitor configurations duringsubsequent pacing pulses.

Upon expiration of the pulse width timer at block 278, the pacing pulseis terminated at block 282. By adjusting the capacitor configurationduring the pacing pulse in response to the pulse amplitude falling to orbelow an amplitude threshold, the truncated amplitude may be maintainedabove a minimum threshold amplitude. The truncated amplitude may besampled to verify that the capacitor configuration adjustmentsuccessfully maintained the truncated amplitude above the minimumthreshold amplitude. If the truncated pulse amplitude reaches theminimum threshold amplitude at truncation as determined at block 284,microprocessor 140 may adjust the initial capacitor configuration usedon a subsequent pacing pulse at block 286 to maintain the pulseamplitude a safety margin above the threshold amplitude at truncation ofthe next pulse.

In some examples, pacing control module 102 may be configured to adjustan initial capacitor configuration based on the sampled pulse amplitudebeing greater than an expected maximum threshold. For example, if thetruncated amplitude is greater than an expected maximum threshold, asdetermined at block 284, the capacitance may be higher than necessaryfor the pacing vector impedance. The initial capacitor configuration maybe adjusted for the next pacing pulse by eliminating one capacitor fromthe configuration at block 286. In some examples, the adjustment of theinitial capacitor configuration at block 286 includes repeating animpedance measurement of the pacing vector to re-determine the requiredcapacitance of the capacitor configuration. If the truncated amplitudeis less than or equal to an expected maximum threshold and greater thanthe minimum threshold, the initial capacitor configuration may remainunchanged at block 288 or be set to the final capacitor configurationthat was used to deliver the pacing pulse.

FIG. 11 is a flow chart 300 of a pacing control method performed by IMD14 according to another example. A pacing pulse is started at block 302.The pacing control module 102 starts a pulse width timer and controlsthe LV therapy delivery module 85 to start discharging an initialcapacitor configuration across an extra-cardiovascular pacing electrodevector. The initial capacitor configuration may be a default capacitorconfiguration, a previously selected capacitor configuration, or setbased on a pacing electrode vector impedance measurement.

At block 304, microprocessor 140 determines that the pulse amplitude hasfallen to or below an amplitude threshold after a first portion of thepulse width, such that a capacitor configuration adjustment is neededduring the pacing pulse. In response to the pulse amplitude falling toor below the amplitude threshold, the pacing control module 102 passes anew capacitor configuration to capacitor selection and control module104. At block 306, capacitor selection and control module 104 adds oneor more capacitors to the initial capacitor configuration according toconfiguration data received from pacing control module 102 to produce asecond capacitor configuration different than the initial capacitorconfiguration. The second capacitor configuration includes at least onecapacitor not included in the initial capacitor configuration but mayhave been previously charged, e.g., when the initial capacitorconfiguration was charged. If not previously charged, the capacitorselection and control module 104 enables charging of the one or morecapacitors added in the second capacitor configuration and upon chargecompletion couples the added capacitor(s) to the output signal line byenabling the appropriate switch(es), e.g., one or more of S2 through S6of FIG. 4 assuming at least C1 is included in the initial capacitorconfiguration. The capacitors included in the initial capacitorconfiguration may remain enabled and coupled to output signal line 130or may be disabled upon adding the one or more capacitors of the secondcapacitor configuration to the discharge circuit at block 306.

The LV therapy delivery module 85 continues delivering the pacing pulseusing the second capacitor configuration at block 308, until the pulsewidth expires. After enabling the second capacitor configuration,microprocessor 140 may fetch pulse amplitude sample point(s) during asecond portion of the pacing pulse for comparison to a minimum thresholdat block 310. As long as the sampled pulse amplitude remains greaterthan the minimum threshold, as determined at block 312, the capacitorconfiguration adjustment is deemed adequate for maintaining thetruncated amplitude of the pacing pulse above a minimum threshold. Ifthe pulse amplitude falls below the minimum threshold at block 312,however, an impedance measurement trigger signal may be generated bymicroprocessor 140 at block 314. Control module 80 may control impedancemeasurement module 90 to perform an impedance measurement of the pacingelectrode vector prior to the next pacing pulse.

At block 316, pacing control module 102 sets the initial capacitorconfiguration for the next pacing pulse. If an impedance measurement wastriggered at block 314, the pacing control module 102 sets the initialcapacitor configuration for the next pacing pulse based on the triggeredimpedance measurement. The capacitance required to achieve an RC timeconstant longer than a threshold time interval may be determined usingthe triggered impedance measurement. The new initial capacitorconfiguration is selected according to the determined capacitance.Alternatively, the new initial capacitor configuration may be retrievedfrom a look-up table in memory 82 by matching the measured impedance toa stored impedance range.

If an impedance measurement was not triggered at block 314, the initialcapacitor configuration set at block 316 may be set to the same initialcapacitor configuration as used on the currently delivered pacing pulseor the adjusted capacitor configuration which was used for deliveringthe pacing pulse during a second portion of the pulse width. In thisway, an impedance measurement may be performed only when triggered inresponse to the pacing pulse amplitude falling below a minimum thresholdafter a capacitor configuration adjustment. In various examples, theimpedance measurement may be triggered when the capacitor configurationhas been adjusted a predetermined number of times, e.g., a single time,two times, or other threshold number of times during a single pacingpulse.

Thus, a method and apparatus for delivering pacing pulses usingextra-cardiovascular electrodes have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or combination than the illustrative examples shown anddescribed herein. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

1. A medical device comprising: a therapy delivery module comprising acapacitor array having a plurality of capacitors for producing astimulation pulse; and a stimulation control module coupled to thetherapy delivery module and configured to: select a first capacitorconfiguration comprising a first combination of the plurality ofcapacitors; start a pulse width timer defining a stimulation pulsewidth; control the therapy delivery module to start delivery of thestimulation pulse by discharging the first capacitor configurationacross an electrode vector coupled to the therapy delivery module;obtain a first sampled amplitude of the stimulation pulse during a firstportion of the stimulation pulse width; determine that the first sampledamplitude is less than or equal to a first amplitude threshold; andcontrol the therapy delivery module to select a second capacitorconfiguration comprising a second combination of the plurality ofcapacitors different than the first combination and discharge the secondcapacitor configuration across the electrode vector during a secondportion of the stimulation pulse width in response to the amplitude ofthe stimulation pulse being less than or equal to the first amplitudethreshold.
 2. The device of claim 1, wherein the therapy control moduleadds one or more capacitors of the plurality of capacitors not includedin the first capacitor configuration to select the second capacitorconfiguration.
 3. The device of claim 2, wherein the therapy controlmodule disables at least one of the capacitors of the first combinationof the plurality of capacitors of the first capacitor configuration toselect the second capacitor configuration.
 4. The device of claim 1,wherein the second capacitor configuration comprises the first capacitorconfiguration and at least one capacitor of the plurality of capacitorsthat was not included in the first capacitor configuration.
 5. Thedevice of claim 1, wherein the stimulation control module is furtherconfigured to: determine that the first sampled amplitude is greaterthan the first amplitude threshold; and control the therapy deliverymodule to continue delivering the stimulation pulse using the firstcapacitor configuration in response to the first sampled stimulationpulse amplitude being greater than the first amplitude threshold.
 6. Thedevice of claim 1, wherein the stimulation control module is furtherconfigured to: obtain a second sampled amplitude of the stimulationpulse during the second portion of the stimulation pulse width;determine that the second sampled amplitude is less than or equal to thefirst amplitude threshold; and control the therapy delivery module toselect a third capacitor configuration comprising a third combination ofthe plurality of capacitors different than the first and secondcombinations and discharge the third capacitor configuration across thestimulation electrode vector during a third portion of the stimulationpulse width in response to the second amplitude of the stimulation pulsebeing less than or equal to the first amplitude threshold.
 7. The deviceof claim 1, wherein the stimulation control module is further configuredto determine a number of capacitor configuration adjustments that havebeen made during the pulse width, determine that the number of capacitorconfiguration adjustments exceeds a maximum number of adjustments, andcontrol the therapy delivery module to continue delivering thestimulation pulse using a current capacitor configuration in response tothe number of capacitor configuration adjustments exceeding the maximumnumber of adjustments.
 8. The device of claim 1, wherein the stimulationcontrol module is further configured to detect expiration of the pulsewidth timer and control the therapy delivery module to terminatedelivery of the stimulation pulse upon the detected expiration.
 9. Thedevice of claim 1, wherein the stimulation control module is furtherconfigured to obtain a sampled pulse amplitude at expiration of thestimulation pulse width, determine that the sampled pulse amplitude atexpiration of the stimulation pulse width is one of greater than amaximum threshold or less than a minimum threshold, and adjust theinitial capacitor configuration for a subsequent stimulation pulse whenthe sampled pulse amplitude at expiration of the stimulation pulse widthis one of greater than a maximum threshold or less than a minimumthreshold.
 10. The device of claim 1, further comprising an analog todigital converter (ADC) configured to sample the amplitude of thestimulation pulse at a sampling rate.
 11. The device of claim 10,wherein the ADC is enabled to begin sampling the amplitude of thestimulation pulse at a leading edge of the stimulation pulse.
 12. Thedevice of claim 10, wherein the ADC is enabled to begin sampling theamplitude of the stimulation pulse after a predetermined time delayfollowing a leading edge of the stimulation pulse.
 13. The device ofclaim 10, wherein the sampling rate is variable such that a firstsampling interval toward a beginning of the pulse width is longer than asecond sampling interval toward an end of the pulse width.
 14. Thedevice of claim 1, further comprising a medical stimulation leadincluding at least one electrode electrically coupled to the therapydelivery module.
 15. A method comprising: selecting a first capacitorconfiguration comprising a first combination of a subset of plurality ofcapacitors of a capacitor array; initiating a pulse width timer defininga stimulation pulse width; initiating delivery of a stimulation pulse bydischarging the first capacitor configuration across an electrode vectorcoupled to the capacitor array; obtaining a first sampled amplitude ofthe stimulation pulse during a first portion of the stimulation pulsewidth; determining that the first sampled amplitude is less than orequal to a first amplitude threshold; selecting a second capacitorconfiguration comprising a second combination of the plurality ofcapacitors different than the first combination in response to theamplitude of the stimulation pulse being less than or equal to the firstamplitude threshold; and discharging the second capacitor configurationacross the electrode vector during a second portion of the stimulationpulse width.
 16. The method of claim 15, wherein selecting the secondcapacitor configuration comprises adding one or more capacitors of theplurality of capacitors not included in the first capacitorconfiguration to select the second capacitor configuration.
 17. Themethod of claim 16, wherein selecting the second capacitor configurationcomprises disabling at least one of the capacitors of the firstcombination of the subset of the plurality of capacitors of the firstcapacitor configuration to select the second capacitor configuration.18. The method of claim 15, further comprising: determining that thefirst sampled amplitude is greater than the first amplitude threshold;and continuing delivery of the stimulation pulse using the firstcapacitor configuration in response to the first sampled stimulationpulse amplitude being greater than the first amplitude threshold. 19.The method of claim 15, further comprising: obtaining a second sampledamplitude of the stimulation pulse during the second portion of thestimulation pulse width; determining that the second sampled amplitudeis less than or equal to the first amplitude threshold; selecting athird capacitor configuration comprising a third combination of theplurality of capacitors different than the first and secondcombinations; and discharging the third capacitor configuration acrossthe stimulation electrode vector during a third portion of thestimulation pulse width in response to the second amplitude of thestimulation pulse being less than or equal to the first amplitudethreshold.
 20. The method of claim 15, further comprising: determining anumber of capacitor configuration adjustments that have been made duringthe pulse width; determining that the number of capacitor configurationadjustments exceeds a maximum number of adjustments; and continuingdelivery of the stimulation pulse using a current capacitorconfiguration in response to the number of capacitor configurationadjustments exceeding the maximum number of adjustments.
 21. The methodof claim 15, further comprising: detecting expiration of the pulse widthtimer; and terminating delivery of the stimulation pulse upon thedetected expiration.
 22. The method of claim 15, wherein obtaining thefirst sampled amplitude of the stimulation pulse comprises obtaining thefirst sampled amplitude of the stimulation pulse after a predeterminedtime delay following a leading edge of the stimulation pulse.
 23. Themethod of claim 22, wherein the predetermined time delay comprises halfof the pacing pulse width.
 24. The method of claim 23, furthercomprising: determining that the first sampled amplitude is greater thanor equal to a second amplitude threshold, the second amplitude thresholdbeing larger than the first threshold; and obtaining no further sampledamplitudes for the remainder of the pulse width.